As the C-terminal octapeptide of cholecystokinin represents a putative neurotransmitter in the central nervous system, the membrane-bound enzymes involved in its inactivation were investigated. Two aminopeptidases, involved in the cleavage of enkephalins, and a metalloendopeptidase were identified in extracts of solubilized synaptic membranes. The metalloendopeptidase, which cleaves CCK-8 at the Trp 3 0 -Met 3 1 bond, appeared to be indistinguishable from 'enkephalinase A 1 ' on the basis of its chromatographic behaviour, sensitivity to inhibitors and relative affinities for Met- and Leu-enkephalins. This finding indicates that CCK-8 is inactivated in vitro by the same peptidases as enkephalins.